Japanese Unexamined Patent Application Publication No. 2003-258527 discloses an antenna for mobile communications whose bandwidth in use is increased by using a plurality of radiating conductors. Moreover, Japanese Unexamined Patent Application Publication No. 11-68453 discloses a composite antenna used in a plurality of frequency bands.
FIG. 1 is a perspective view of the antenna described in Japanese Unexamined Patent Application Publication No. 2003-258527. This antenna mainly includes a first dielectric substrate 21 and a second dielectric substrate 22. A ground electrode is formed on substantially the entire bottom surface of the first dielectric substrate 21, and a first radiating conductor 23, a second radiating conductor 24, and a third radiating conductor 25 each having an L shape are formed on either or both of the surfaces of the second dielectric substrate 22. The total length of the first radiating conductor 23 is slightly larger than an eighth-wavelength of the central frequency in the frequency band in use, and the length of the second radiating conductor 24 is slightly smaller than that of the first radiating conductor. Furthermore, the total length of the third radiating conductor 25 is substantially a quarter-wavelength of the central frequency in another frequency band in use whose frequencies are higher than those of the above-described frequency band.
FIG. 2 is a schematic view of the composite antenna described in Japanese Unexamined Patent Application Publication No. 11-68453. This composite antenna 10 includes main elements (11, 14) whose first ends serve as feeding points and sub-elements (13, 16) formed by folding back second ends of the main elements such that the feeding ends serve as open ends. The plurality of substantially U-shaped folded antennas A, B each correspond to a frequency band in use, and the main elements (11, 14) and the sub-elements (13, 16) protrude from a ground plane 3.
Since the antenna described in Japanese Unexamined Patent Application Publication No. 2003-258527 as shown in FIG. 1 has a structure in which the substrate having the radiating electrodes formed thereon is positioned upright on another substrate (i.e., motherboard), the antenna cannot be incorporated into mobile communication devices such as mobile phone units whose thickness needs to be reduced.
Moreover, although the composite antenna described in Japanese Unexamined Patent Application Publication No. 11-68453 as shown in FIG. 2 can be used in two frequency bands, the antenna is not suitable for three frequency bands. That is, even when three sub-elements are provided for the main elements by folding back the first ends of the main elements serving as the feeding points based on a similar concept, three resonance characteristics may be degraded by interference between the sub-elements. As a result, a composite antenna available for three frequency bands may not be obtained.
In view of the shortcomings of the above-discussed prior art, an embodiment of a multi-resonant antenna consistent with the claimed invention includes three independent resonance characteristics that are not degraded and the antenna is operable in three frequency bands.
In order to solve the above-described problems, a multi-resonant antenna consistent with the claimed invention has the following structure.
(1) The multi-resonant antenna includes a first electrode with an open end having a length corresponding to a first frequency band and extending from a feeding portion in a first direction along the periphery of a substantially rectangular area; a second electrode with an open end having a length corresponding to a second frequency band, the second frequency band being higher than the first frequency band, and the second electrode extending from the feeding portion in a second direction opposite to the first direction along the periphery of the substantially rectangular area; and a third electrode with an open end having a length corresponding to a third frequency band, the third frequency band being intermediate between the first and second frequency bands, and the third electrode extending from a predetermined point of the first or second electrode or from the feeding portion along the first electrode inside the substantially rectangular area surrounded by the first and second electrodes, the open end of the third electrode being closer to the open end of the first electrode than to the open end of the second electrode.
(2) The open end of the third electrode is closer to the open end of the first electrode than to a midsection in a longitudinal direction of the first electrode when viewed from the feeding portion.
According to the embodiment, the third electrode is disposed or nested inside the first and second electrodes so as to be adjacent to the first electrode, which is longer than the second electrode. According to the embodiment, the antenna can be well matched at the resonant frequency corresponding to the third electrode.
In addition, since the third electrode does not significantly affect the two resonance characteristics by the first and second electrodes, desired three resonance characteristics can be obtained.
The following description of various aspects and embodiments will further clarify the above-mentioned features and advantages.